Remember that appearance is not everything. Sure how someone looks leads us to many conclusions, but as far as vocalists are concerned it is something that leads to maintaining their style and identity. It is in keeping up with their image that they present themselves the way that they do.
Make sure that when you are hiring a vocalist that they are able to perform all of the songs that you would like performed. But also keep in mind that you want them to perform the songs in the style that you want as well. It is a matter of style that you are concerned with here.
Vocalists are what make the song sound how it is. Each different vocalist can put their own spin on the way it will be portrayed. Different genre songs performed by different style vocalists can very easily convey different meanings. Check into what type and style of vocalist you want.
